The present invention relates to an improved method of coating multilayer liquid packs on moving webs involved in the manufacture of photographic elements. More particularly, the present invention involves the coating of a non-gelatin overcoat over a topmost gelatin layer in a photographic element. In one embodiment, a processing-solution-permeable overcoat is simultaneously coated with the emulsion layers onto a photographic substrate, which overcoat becomes water and stain resistant in the photochemically processed product.
In many instances it is desired to coat the surface of an object with a plurality of distinct, superposed layers (collectively, the plurality of layers is also known as a coating pack). In the manufacture of photographic elements, such as photographic film, wherein a number of layers (up to ten or more) of different photographic coating compositions must be applied to a suitable support in a distinct layered relationship, the uniformity of thickness of each layer in the photographic element must be controlled within very small tolerances. Common methods of applying photographic coating compositions to suitable supports involve simultaneously applying the superposed layers to the support. Typically, a coating pack having a plurality of distinct layers in face-to-face contact is formed and deposited on the object so that all the distinct layers are applied in a single coating operation. In the photographic industry, several such coating operations may be performed to produce a single photographic element. Several methods and apparatus have been developed to coat a plurality of layers in a single coating operation. One such method is by forming a free falling, vertical curtain of coating liquid which is deposited as a layer on a moving support Exemplary xe2x80x9ccurtain coatingxe2x80x9d methods of this type are disclosed in U.S. Pat. No. 3,508,947 to Hughes, U.S. Pat. No. 3,632,374 to Grieller, and U.S. Pat. No. 4,830,887 to Reiter. xe2x80x9cBead coatingxe2x80x9d is another method of applying a plurality of layers to a support in a single coating operation. In typical bead coating techniques, a thin liquid bridge (a xe2x80x9cbeadxe2x80x9d) of the plurality of layers is formed between, for example, a slide hopper and a moving web. The web picks up the plurality of layers simultaneously, in proper orientation, and with substantially no mixing between the layers. Bead coating methods and apparatus are disclosed, for example, in U.S. Pat. Nos. 2,681,294 and 2,289,798.
U.S. Pat. Nos. 5,306,527 and 5,310,637 disclose methods of reducing the tendency toward formation of ripple imperfections in the coating of multilayer photographic elements. In U.S. Pat. No. 5,310,637, it is stated that ripple or ripple imperfection is defined for the purposes of this invention as a layer thickness nonuniformity resulting from wave growth at the fluid-fluid interfaces of a plurality of layers due to a hydrodynamic instability of the gravity-induced flow of the plurality of layers on a coated web. The patent theorizes that ripple imperfections arise when there are viscosity differences between adjacent layers of multilayer coating packs. These viscosity differences can be introduced in a variety of ways, including initial viscosity differences between the various layers as delivered to the web or changes in relative layer viscosities from thermal effects after the layers are coated on a web. Another theorized cause was interlayer mass transport of solvent, for example, in the coating of photographic elements, where adjacent layers often contain varying amounts of gelatin. It was thought that these differences cause water diffusion between the layers which, in turn, can significantly alter the resulting viscosities of the individual layers after they are coated on the web. In this way, viscosity disparities between layers may be introduced on the web for layers which were originally coated at nominally equal viscosities. It was also stated that an osmotic pressure difference between adjacent layers drives interlayer water diffusion in gelatin-containing multilayer coating packs, such as commonly used in the photographic industry and that, in many cases, osmotic pressure differences may result from significant differences in the layer concentrations of gelatin and other addenda. The patent further teaches that the tendency toward the formation of ripple imperfections in multilayer coatings can be reduced by controlling the gelatin concentration of adjacent layers. For example, in a multilayer coating pack having upper, middle, and lower gelatin-containing layers, respectively, the patent concludes that the tendency toward the formation of ripple will be greatly reduced if the middle layer has a gelatin concentration within three weight percent of the gelatin concentration of each of the upper and lower layers and each of the layers has a viscosity which differs from a norm by no more than fifteen percent. U.S. Pat. No. 5,306,537 teaches methods of coating multilayer gelatin based coating packs in which the compositions are determined according to a given formula to keep the ripple value below 35. This formula includes maintaining certain viscosity ratios between adjacent layers. In a gelatin-based coating, maintaining similar viscosities is typically achieved by maintaining similar gelatin concentrations. As a result, inherently the osmotic pressures are naturally kept close and prevent instability problems.
In both bead coating and curtain coating methods, it is necessary to set and/or dry the layered coating after it has been applied to the support. To accomplish this, the web is typically conveyed from the coating application point to a chill section. Subsequently, the web is conveyed through a series of drying chambers after which it is wrapped on a winder roll. Space constraints for the coating machine, cost considerations, and flexibility of design may dictate that one or more inclined web paths be present in conveying the coated substrate from the coating point to the chill section and drying chambers.
Advancements in coating technology have led to increased numbers of layers coated at each coating station, increased total pack thickness per station, thinner individual layers, use of rheology-modifying agents, and the development of new, sophisticated chemistries. In addition, a multilayer photographic coating can consist of sensitizing layers and/or additional, non-imaging, layers. As a result, the chemical composition of the multilayer coating pack is often markedly different from one layer to the next.
A number of patents have been directed to water-resistant protective coatings that can be applied to a photographic element prior to development. For example, U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing water- and fingerprint-resistance by coating the light-sensitive layer, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. More recently, U.S. Pat. No. 5,853,926 to Bohan et al. discloses a protective coating for a photographic element, involving the application of an aqueous coating comprising polymer particles and a soft polymer latex binder. This coating allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. Again, however, the hydrophobic polymer particles must be fused to form a protective coating that is continuous and water-impermeable.
U.S. Pat. No. 5,856,051 describes the use of hydrophobic particles with gelatin as the binder in an overcoat formulation. This invention demonstrated an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. The hydrophobic polymers exemplified in U.S. Pat. No. 5,856,051 include polyethylene have a melting temperature (Tm) of 55 to 200xc2x0 C., and are therefore capable of forming a water-resistant layer by fusing the layer at a temperature higher than the Tm of the polymer after the sample has been processed to generate the image. The coating solution is aqueous and can be incorporated in the manufacturing coating operation without any equipment modification. Again, however, fusing is required by the photofinishing laboratories to render the protective overcoat water-resistant. Similarly, commonly assigned U.S. Ser. No. 09/353,939 and U.S. Ser. No. 09/548,514, respectively, describe the use of a polystyrene-based material and a polyurethane-based material, with gelatin as the binder, in an overcoat for a photographic element, which overcoat can be fused into a water resistant overcoat after photographic processing is accomplished to generate an image.
Commonly assigned U.S. Ser. No. 09/235,436 discloses the use of a processing solution permeable overcoat that is composed of a urethane-vinyl copolymer having acid functionalities. Commonly assigned U.S. Ser. No. 09/235,437 and U.S. Pat. No. 6,194,130 B1 disclose the use of a second polymer such as a gelatin or polyvinyl alcohol to improve processibility and reduce coating defects. However, it has been found that in order to achieve the functionality of water impermeability, it is undesirable to have gelatin in the overcoat, since the second polymer is expected to exit the imaging element upon processing, and gelatin, being crosslinkable, does not exit the coating. Commonly assigned U.S. Ser. No. 09/621,267 discloses the use of a processing solution permeable overcoat that is composed of various non-gelatin containing hydrophobic polymers in combination with a hydrophilic polymer.
While the prior art has disclosed imaging elements with a processing permeable overcoat that is rendered water impermeable, and the materials used to prepare such overcoats, it has not been specific in how these imaging elements have been prepared. The desired overcoat may be applied in several possible methods. It may be applied to a imaging element that is previously coated with all layers except the overcoat. In such a case, the overcoat may be applied as a single layer. It also could be applied in a single coating operation, in a tandem method. In this case all the layers, except the desired overcoat can be applied at a first station in the coating machine. The web is then dried and run through a second coating station, without winding it up, where the overcoat is applied.
The most preferred method for coating an overcoat is at a single coating station, along with the other imaging layers. This is typically accomplished with gelatin overcoats using a slide hopper where multiple solutions are layered without mixing. The layered solutions are then deposited on the web either by bead coating or by dropping it as a curtain onto the web.
The present invention addresses this problem and discloses a method of reducing the likelihood and severity of coating non-uniformities in coating multilayer liquid packs in the photographic industry. In particular, it has been found that when attempting to simultaneously coat at least one non-gelatin-containing layer adjacent to a gelatin-containing layer can often result in coating non-uniformities.
According to another more specific aspect of the invention, it would also be desirable to allow a polymeric latex protective overcoat to be coated simultaneously with underlying emulsion layers in a so-called single pass operation, during manufacture of a photographic imaging element, as compared to a so-called xe2x80x9ctwo-passxe2x80x9d coating operation. Thus, it would also be desirable to obtain an imaging element comprising an overcoat that is process-permeable during photoprocessing and which can be converted to a water-resistant protective overcoat for the imaged element, which water resistance is not lost or decreased when the overcoat is simultaneously coated with the emulsion layers. It would be further desirable if this could be accomplished without the addition of laminating or fusing steps, without the need for high temperature fusing, and preferably with minimal or no additional equipment to carry out photoprocessing.
In accordance with the present invention, it has been discovered that coating non-uniformities can occur in multilayer coating packs when there are osmotic differences between a non-gelatin-containing layer and a gelatin-containing layer, which non-gelatin-containing layer is overlying and adjacent to the gelatin-containing layer, after coating those layers on a moving web. The present invention enables the design and use of coating compositions that exhibit a greatly reduced tendency toward the formation of coating non-uniformities. The present invention helps obviate a significant coating problem that will become increasingly prevalent, especially in the photographic industry, stemming from the development and use of new, non-gelatin-containing layers.
In particular, this invention relates to a method of simultaneously coating at least one non-gelatin-containing layer over and adjacent to a topmost gelatin-containing layer, which layered mass further comprises at least one silver-halide emulsion layer, wherein the osmotic pressure of the of the non-gelatin layer is not more than 30 percent less than the osmotic pressure of the gelatin-containing layer, as measured by standard device. More than one non-gelatin-coating layer can overlie the topmost gelatin-containing layer, and the layers can be on the frontside or backside of the photographic element. In a preferred embodiment, the osmotic pressure of the non-gelatin-containing layer is less than the osmotic pressure of the gelatin-containing layer.
It has been found that polymer latex coating formulations will commonly have low osmotic pressures which account for coating stability problems. Without being bound by theory, it is believed that this happens because of osmotic pressure mismatches between adjacent layers result in water moving from one layer to another. This results in changes in the concentrations of components in the layers, in turn resulting in viscosity changes that can cause coating instabilities as described in prior art. In polymeric systems, one primary way of controlling osmotic pressure is with the addition of a water soluble polymer. Along with gelatin-containing layers, multiple polymer layers may be coated simultaneously with the purpose of imparting different physical properties from each layer. One example is one layer for a moisture barrier and one for a high gloss surface.
In another aspect of the invention, the method is used to simultaneously coat a photographic imaging element in which the overcoat can be converted into a water-resistant coating. In particular, it has been found that stain resistance and/or water resistance of an imaged element having a protective overcoat, which is the topmost non-gelatin layer on the frontside of the photographic element, can be obtained or enhanced, when the overcoat (nascently protective) is coated simultaneously with the gelatin-based emulsion layers, by controlling the osmotic pressure of the layers so that the osmotic pressure of the non-gelatin-containing layer is not more than 30 percent less than the osmotic pressure of the gelatin-containing layer, as measured by a standard device described below. For example, such a photographic element may comprise a support, at least one silver-halide emulsion layer superposed on the support, and overlying the silver-halide emulsion layer, a processing-solution-permeable protective overcoat composition that can be incorporated into or coated on the imaging element during manufacturing and that does not inhibit photographic processing. The non-gelatin containing layer according to the present invention comprises water dispersible polymer particles in a latex form or a conventional colloidal dispersion of a hydrophobic film forming material along with a water soluble polymer. The presence of a water soluble component that is substantially washed out during processing allows photographic processing to proceed at an acceptable rate. The washing out of the water soluble component facilitates the coalescence of the polymer particles to form a continuous protective overcoat in the final product.
In one embodiment of the invention, the overcoat composition applied to the imaging element comprises 30 to 95 weight percent, based on the dry laydown of the overcoat, of water-dispersible polymer particles having an average particle size of between 0.01 to 0.5 micrometers, said water-dispersible polymer being characterized by a Tg (glass transition temperature) of between xe2x88x9240 and 80xc2x0 C. In general, the overcoat composition preferably contains a water-soluble, hydrophilic polymer that is typically noncrosslinked to facilitate its washing out during processing and, at least to some extent, to facilitate the coalescence of the water-dispersible polymer particles. Preferably, the overcoat formulation is substantially gelatin-free, comprising less than 5% crosslinked gelatin by weight of solids.
In another embodiment of the invention, the overcoat composition applied to the imaging element comprises 5 to 70% by weight of solids of water-soluble hydrophilic polymer such that more than 30 weight percent of the water-soluble polymer is washed out during photographic processing; wherein the weight ratio of the water dispersible polymer particles to the non-crosslinked water soluble polymer is between 60:40 to 85:15 and whereby the overcoat forms a water-resistant overcoat after photoprocessing without fusing.